These may be broadly divided into three classes :-

1. ;High Speed Steels.

2. ;Special Alloy Steels.

3. ;Carbon Tool-Steels.

Taking these in turn :-

High Speed Sleets

There is no exaggeration in saying that the discovery of high - speed steel revolutionised machine-shop practice. For machining, it was so manifestly superior to carbon tool-steel, and to the old self-hardening steel, that before many years had passed it was in vogue wherever men used a lathe. So wide-spread has its use now become that, even in a little factory where half-a-dozen lathes are crowded into space not much bigger than a fair-sized office, the best brands of highspeed steel are to be seen at work.

What did high-speed steel do ? Exactly what raised it to this pre-eminent position amongcutting-steels ? That is soon explained. It was found that by adding to carbon-steel a percentage-originally 12 to 13 per cent., but now ranging from 14 to 18 per cent, and upwards-of the metallic element tungsten, and a smaller proportion of other elements, a property known as " red-hardness "was given to the resultant alloy.

This quality of " red-hardness " enabled high-speed steel to retain a keen cutting edge when heated to a temperature which would infallibly have crippled the ordinary carbon-steel lathe-tool. In consequence, work could be done faster, much faster. Also, although the high-speed steel tool became almost red-hot while in use, it continued to cut and was, after cooling, by no means less efficient, and could be used again and again.

Obviously the high cost of this steel was more than compensated by the longer life, swifter and better service, and therefore increased output, which it gave. Wherever works were producing at high pressure, it was employed with uniform success.

Practice now divides high-speed steels into three main varieties or qualities, governed by the percentage of tungsten, etc., in their composition.

Edgar Allen Stag Extra Special is a super high-speed steel which has its special applications. It is costly, but in spite of this can be used economically in exceptional cases, such as turning exceptionally hard worn-steel tyres.

For the same work, where the duty to be performed is not so severe, or where a steel a little less costly is required, the Edgar Allen Stag Special quality (18 per cent, tungsten) is most effective.

For ordinary work, the Edgar Allen Stag Air-hardening quality (14 per cent, tungsten) is found economical. The results obtained with it depend largely on (1) the nature of the material to be cut, (2) the character and state of repair of the machines, (3) the available power for driving them, and (4) the intelligence and skill of the men who ^ make and use the tools.

The whole art of working highspeed steel successfully depends upon the simple process of heating it. The following instruction should never be forgotten :-

Warm the steel before you place it actually in the fire, then heat up slowly, thoroughly and evenly.

If this simple hint is followed, it is practically impossible to spoil the steel. The reason for stressing this point is that high-speed steel is much denser than ordinary steels. Heat takes longer to penetrate it, and consequently, if a bar is placed in a hot fire, the outside expands more quickly than the interior, thus causing an internal " clink." To the eye the bar appears as before, and even when the tool is hardened and ground, no flaw can be detected. When put to work, however, the tool breaks, ' the fractured surfaces showing those 14 cup - and - egg " - shape appearances so characteristic of careless heating.

Forging

Always heat the steel before cutting it. Forge it at an orange colour (1000° C.) ; re-heat when forged, and set aside to cool. Do not omit the re-heating ; it takes away the forging strains. When cold the tool may be rough-ground to shape if necessary.

Hardening

Harden by re-heating only the cutting edge of the tool as Fig. 195, to a white heat (1250-1300° C.) and cool out quickly in either a strong blast of cold air. which must be perfectly dry, whale oil, or one of the standard proprietary brands of good-class oil. Air-hardening is recommended for cutting-tools in preference to oil-hardening. Two hints should be noted :-

Fig. 195.

Heat up very carefully till the forging heat is passed, then more rapidly to the final heat. If you cool in oil, cool only the cutting edge and keep it moving.

If the tool is hardened too far up, trouble in several directions may be confidently expected.

Whether high-speed steel should be hardened in a blast of air or quenched in oil depends on the convenience with which either can be carried out, and also to some extent on the mass of the tool to be hardened. If the tool is of small section, air-hardening is possibly preferable, but if the tool section is very heavy, oil-hardening would probably give more reliable results, the cooling-power of the oil being more effective when a heavy mass is undergoing treatment.

Hardening Milling-Cutlers

To preserve the cutting-edges from corrosion, etc., the cutters must be heated up in a salt bath, or if in a furnace, in a closed receptacle, and may be covered with some kind of silica paint as a further protection. They must be heated to the temperature already given, and the precautions against too rapid heating cannot be too strictly observed. Cool out in air, fish-oil, lard-oil, whale-oil, or cotton seed-oil. The performance of milling-cutters can sometimes be improved by tempering.

Tempering

To get the maximum hardness and toughness in high-speed steel after hardening, it should be tempered at 580° to 600° C. and cooled in air. It should be brought to the desired drawing temperature either in a salt bath or furnace, under strict control, and care should be taken that the 600° C. is not exceeded. The tools should be well warmed before placing in the salt bath or furnace, otherwise cracks may be produced. They should be held from ten to thirty minutes at the drawing temperature, according to the size of the tools, and may be air-cooled or oil quenched after drawing. This drawing process should not be attempted unless the furnace is under strict pyrometric control, because the limits of temperature which give the steel these added benefits are very narrow, and any temperature-increase progressively softens the steel.

When sufficient time has elapsed for a uniform heat to penetrate to the centre, coo! off as slowly as possible until quite cold. If the heating has been done in a furnace, the furnace should be allowed to cool down with the steel in it. If the heating has been done in a forge, the receptacle should be covered with red-hot coals and left till cold. A ready method, which answers well enough for many purposes, is to heat the steel to the heat previously mentioned, and quickly bury it in lime, which only allows it to cool very slowly.